1. Field of the Invention
The invention provides an improved miniature refrigeration system or cryocooler for cooling an element to a cryogenic temperature, e.g. less than 200° K. but usually 77° K. or below, and for maintaining the element at the cooled temperature for an extended period. In particular, the invention provides improvements to a DC motor used to drive a refrigeration gas compressor, improvements to the refrigeration gas compressor and improvements to elements of the drive coupling between the DC motor and a compressor piston and a regenerator piston.
2. Description of the Prior Art
Miniature refrigeration devices based on the Stirling-cycle are used in infrared (IR) imaging systems to maintain an IR detector at a constant cryogenic temperature during operation. Miniature refrigeration devices may also be used in other fields, e.g. for maintaining superconductive materials at a constant cryogenic temperature during operation. However, the improvements described herein are more generally usable in DC motors and compressors used for any application that may require increased operating efficiency and improved reliability.
A Stirling cycle cryocooler is described in U.S. Pat. No. 4,858,442 to Stetson, commonly assigned herewith, and incorporated herein by reference. In particular, Stetson describes a miniature cryocooler system that includes a DC motor driving each of a compression piston and a regenerator piston. Prior art DC motors used in commercial cryocoolers include a drive shaft rotating about a rotation axis and having a crank pin extending from an end face of the drive shaft parallel to the rotation axis and rotating eccentrically about the rotation axis. A drive coupling attached to the crank pin moves eccentrically about the shaft rotation axis generating linear motion for driving the compressor and regenerator pistons along a linear path. A flexible vane is linked between the drive coupling and each of the compression and regenerator pistons. Each flexible vane comprises a flat thin flexible spring constrained at its end by the drive coupling and a clamp on the piston being driven. During operation, each flexible vane bends along its longitudinal axis as it pushes and draws each piston reciprocally along the linear axis formed by the corresponding compression and regenerator cylinders. One mode of catastrophic cryocooler failure occurs when a flexible vane fractures or is bent beyond the yield limit of the material and becomes permanently bent. Such a failure may occur when a piston stalls within a cylinder. Other flexible vane failures may occur when misalignment of the vane may induce twisting and in-plane bending, perpendicular to the desired bend axis, during each cycle. This condition can eventually lead to fatigue failure in the flexible vane. Flexible vane failures usually occur when bending and or more complex stresses exceed the fracture or yield stress limit of the material. This usually occurs at stress concentration areas such as at sharp corners, holes or at locations where the vane is constrained.
As detailed in Stetson, the compression strokes of the compression and regenerator pistons occur 90° out of phase. In addition, the compression piston and regenerator piston each require a different linear drive force. Accordingly, the torque load on the DC motor is continuously varying during each rotation of the drive shaft. This causes the motor shaft to bend and applies non-uniform loading to the motor bearing. As a result, shaft bending generates unwanted vibration and non-uniform bearing loads increase bearing wear. Both conditions ultimately reducing the useful life of the motor bearings and degrade the operational efficiency of the cryocooler system. Other problems of prior art cryocooler devices lead to a shortened useful life. In particular, the drive forces applied to compression and regenerator pistons are directed along a continuously varying force direction by the flexible vane. In particular, the piston drive forces are almost never applied along the piston motion axes. As a result, the force direction always tends to force of each piston against the side wall of its corresponding cylinder. Accordingly, piston and cylinder mating surface wear eventually leads to an increased clearance in the gap between the piston and cylinder walls, piston vibration and audible noise. In addition the constant variation in force direction further increases the non-uniformity of the single rotation torque load required by the DC motor. All of these problems may contribute to a decrease in cooling power; an increase in electrical power used, and heat generation within the motor, compressor and regenerator. In addition, as a result of wear and degradation of individual components, the operating efficiency of the cryocooler tends decrease over its operating life and may ultimately lead to premature system failure. In life tests of prior art cryocooler devices, the most common failure modes were found to be failed motor bearings, excessive piston to cylinder clearance, and bent or fractured flexible vanes. In general, as the system components continue to wear high temperature operation, lubrication breakdown, cold welding between moving elements, contaminants, and excess play between moving elements all tend to accelerate performance decline.
Since it is desirable to increase the operating efficiency, reliability and useful life of the cryocooler system and since the most common failure modes are motor bearing, piston to cylinder interface, and flexible vane failures, solutions for addressing these specific problems are addressed by the present invention.
Recently the demands of customers and an increase in competition in the market have provoked a desire to improve performance and the reliability of miniature refrigeration devices. In addition, new applications for miniature refrigeration devices in commercial markets have motivated manufacturers to attempt to lower prices in order to capture previously unavailable market share. With respect to miniature refrigeration device performance, reducing power consumption is a constant goal of designers since most miniature refrigeration units are battery operated or operated in systems that require strict power conservation, e.g. aircraft and space vehicles. Accordingly, there is a need in the art to improve the efficiency of converting electrical power to cooling power to extend battery life. There is also a need in other applications to provide higher cooling capacity and faster cool down times. In some critical imaging applications there is a need to reduce vibration. In general there is a need to improve the reliability of commercial cryocoolers and as the number of commercial applications increase there is a growing need to make system repairs. e.g. to replace worn motor bearing. The needs of the art detailed above are addressed in nearly every aspect of the present invention as will be understood after reading the detailed description.
Other market factors, especially the potential for capturing new commercial markets and the emergence of global competition, have provoked attempts to reduce the manufacturing cost of a miniature refrigeration device. Manufacturing costs are reduced by reducing the cost of materials and by reducing the cost of labor. Part and material cost reductions can be accomplished by eliminating parts and incorporating increased functionally into existing parts. Labor cost reduction may be reduced by eliminating assembly and process steps and especially those that require specialized labor skills. The need to reduce part count and labor is addressed in several aspects of the present invention as will be understood after reading the detailed description.